Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, which electrically couple the pacemaker to the heart.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system applies pacing pulses to and senses cardiac activity in only one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
Single chamber atrial pacing with either a single-chamber or dual-chamber device provides atrial pacing when required during atrial bradycardia. It is used in patients that have intact AV conduction. The resulting atrial synchrony enables ventricular activity to track atrial activity to more closely approximate normal response to exercise or other physiological activity.
Many pacemakers incorporate a physiologic sensor. Such sensors are employed to detect the patient's degree of activity for regulating the heart rate. Hence, as the patient becomes more active, requiring increased cardiac output, the stimulation rate of the pacemaker is increased. When the patient becomes less active, requiring reduced cardiac output, the stimulation rate of the pacemaker is in turn decreased.
Single-chamber atrial pacemakers implement two main timing intervals to support their operation. These intervals are referred to as the refractory period and the atrial escape interval. The refractory period is the time from an atrial pacing pulse or a detected P wave to after the T wave of the ventricle. During this time, the device will not respond to sensed activity to prevent a far field R wave or T wave from being detected as an intrinsic P wave.
The atrial escape interval is the time from a detected P wave or an atrial pacing pulse to when a next atrial pacing pulse is to be delivered absent a preoccurring P wave. This pacing is referred to as atrial demand pacing.
Sensing of far field ventricular activations (R wave) in a single atrial channel is a major problem in single-chamber atrial devices or in dual-chamber devices operating in a single chamber atrial pacing mode. In such devices, it is not easy to distinguish far field ventricular activations from true atrial activity because these devices do not utilize a ventricular channel. Traditionally, such devices utilize various methods in an effort to minimize the impact of the sensing of the far field activity. These methods include refractory time, rate modulated refractory time, or absolute refractory time, for example. All these methods attempt to block the far field ventricular signal or treat it as a signal which should be discarded. If the device fails to block the far field ventricular signal, the signal can then inhibit the pacing of the device and result in a non-output condition when an output may be required.
The amplitude and timing of the far field ventricular signal generally vary with lead location and AV node conduction sufficiency, which is modulated by neurotransmitters. This makes it difficult to determine the proper refractory time to be used. The AV conduction also depends upon whether there is an intrinsic atrial event or a paced atrial event. The difference can be dramatic. Incorrect sensing of far field ventricular events can lead to an incorrect diagnosis and/or a failure to deliver therapy. In addition, pacing induced heart block can result from increased pacing rate, particularly with modern atrial over-drive therapy where the AV node conduction may not be able to accommodate the pacing rate increases demanded by the therapy.
Thus, the present invention addresses these issues in order to fully utilize the advantages of single-chamber atrial pacing devices and the atrial pacing therapies obtainable therewith.
As will be seen hereinafter, the invention provides a device and method which utilizes active searching for far field ventricular activity to enable the proper atrial refractory time to be determined dynamically and an upper pacing rate limit to be established.